1. Field of the Invention
The invention relates to a method of displaying an image on a display device in a plurality of sub-fields, each sub-field for outputting a respective illumination level by the display device, wherein the image includes a plurality of pixels each having a respective intensity value from a set of intensity values and wherein at least one of these intensity values can be generated by a plurality of combinations of the sub-fields, the method comprising the steps of selecting, for a particular pixel, a combination of sub-fields in conformance with its intensity value, and sending a representation of the selected combination to the display device for displaying the particular pixel.
The invention further relates to an image display unit for displaying an image on a display device in a plurality of sub-fields, each sub-field for outputting a respective illumination level by the display device, wherein the image includes a plurality of pixels each having a respective intensity value from a set of intensity values and wherein at least one of these intensity values is generated by a plurality of combinations of the sub-fields, the image display unit comprising selection means for selecting for a particular pixel, a combination of sub-fields in conformance with its intensity value, and sending means for sending a representation of the selected combination to the display device for displaying the particular pixel.
The invention further relates to an image display apparatus comprising such an image display unit.
2. Description of the Related Art
U.S. Pat. No. 5,841,413 describes a plasma display panel driven in a plurality of sub-fields. A plasma display panel is made up of a number of cells that can be switched on and switched off. A cell corresponds with a pixel (picture element) of the image that is to be displayed on the panel. In the operation of the plasma display panel, three phases can be distinguished. The first phase is the erasure phase in which the memories of all cells of the panel are erased. The second phase is the addressing phase, in which the cells of the panel that are to be switched on are conditioned by setting appropriate voltages on their electrodes. The third phase is the sustain phase, in which sustain pulses are applied to the cells which cause the addressed cells to emit light for the duration of the sustain phase. The plasma display panel only emits light during this sustain phase. The three phases together are called a sub-field period or simply a sub-field. A single image, or frame, is displayed on the panel in a number of successive sub-field periods. A cell may be switched on for one or more of the sub-field periods. The light emitted by a cell in the sub-field periods in which it was switched on, is integrated in the eye of the viewer who perceives a corresponding intensity for that cell. In a particular sub-field period, the sustain phase is maintained for a particular time resulting in a particular illumination level of the activated cells. Typically, different sub-fields have a different duration of their sustain phase. A sub-field is given a coefficient of weight to express its contribution to the light emitted by the panel during the whole frame period. An example is a plasma display panel with 6 sub-fields having coefficients of weight of 1, 2, 4, 8, 16 and 32, respectively. By selecting the appropriate sub-fields in which a cell is switched on, 64 different intensity levels can be realized in displaying an image on this panel. The plasma display panel is then driven by using binary code words of 6 bits each, whereby a code word indicates the intensity level of a pixel in binary form.
In driving a plasma display panel, the frame period, i.e., the period between two successive images, is divided into a number of sub-field periods. During each of these sub-field periods, a cell may or may not be switched on, and the integration over the sub-field periods results in a perceived intensity level of the pixel corresponding with this cell. Instead of displaying a pixel integrally, on a plasma display panel, the pixel is displayed as a series of sub-pixels shifted in time with respect to each other. This may cause artifacts if the eyes of the viewer move. Then, it appears as if the sub-pixels do not originate from a single position and a blurring effect occurs. Furthermore, artifacts may occur in case the images show a moving object. The movement needs to be taken into account when displaying the object in a number of sub-fields. For each next sub-field, the object must be moved a little. Motion compensation techniques are used to calculate a corrected position for the sub-pixels in the sub-fields. In some circumstances, the motion compensation is not fully reliable and may produce erroneous results, e.g., in an area of the image with little detail. The erroneous results lead to motion compensation where this should not be done. This gives so-called motion artifacts which are very visible.
An artifact is most noticeable if two neighboring pixels have a small difference in intensity level while, for one of the pixels, the sub-field with the largest coefficient of weight is on and, for the other of the pixels, this sub-field is off. In case of the example of the binary code above, the code word for one pixel has the most significant bit on and the code word for the other pixel has the most significant bit off. Any error in the calculated position of a sub-field, i.e., any motion artifact involving these pixels, will then give a relatively large artifact in the displayed image. The device described in U.S. Pat. No. 5,841,413 tries to mitigate these artifacts by restricting the code words that are used. This known device employs more sub-fields than necessary for realizing the required set of intensity values. The resulting set of code words for expressing the intensity value is redundant, i.e., for a given intensity value, more than one code word is avaiable. From this redundant set, a subset is created whereby those code words are selected that give the least differences in the most significant bit for expressing a difference between the intensity values. This subset is created by searching the original set and determining what the effect on the artifacts may be for a difference between a given code word and each of the other code words.
It is an object of the invention to provide a method as described in the preamble with an improved reduction of artifacts. This object is achieved, according to the invention, in a method which is characterized in that the combination of sub-fields for the particular pixel is selected on the basis of the combination of sub-fields selected for at least one other pixel of the image. This makes it possible to select, for the particular pixel, the best combination of sub-fields from among the possible combinations taking into account the actual content of the image to be displayed. The best combination of sub-fields is that one where any occurring artifact is as small as possible. The selection of sub-fields for the particular pixel on the basis of actual content is better than the selection made in the known method, where the combination is selected only on the basis of the comparison of the combinations themselves.
An embodiment of the method according to the invention is characterized in that the combination of sub-fields for the particular pixel is selected to contain the same sub-fields as the combination of sub-fields for the at least one other pixel, such to the extent possible and with preference for the sub-field for outputting the highest illumination level. Hence, any difference between the intensity levels of the particular pixel and the other pixel will be realized as much as possible with sub-fields for the low illumination levels. Therefore, the two combinations will have the same sub-fields for the high illumination levels whenever possible. Or differently phrased, the result is that the most significant bits of the code words representing the pixel intensity values are identical to each other whenever possible. If the sub-fields for the higher values are identical to each other, then any artifact will be caused by a sub-field of lower intensity and will thus be less visible.
An embodiment of the method according to the invention characterized in that the pixels of the image are received in a serial manner and wherein the combination of sub-fields for the particular pixel is selected on the basis of the combination of sub-fields selected for at least one other pixel received prior to the particular pixel. Since the selection of the combination of sub-fields is made on the basis of one or more earlier received and processed pixels, it is not necessary to store the image in a memory for determining the combination of sub-fields. Such a memory would be necessary if the combination of sub-fields for a particular pixel was determined on the basis of a pixel that was not yet received.
An embodiment of the method according to the invention is characterized in that the pixels of the image are organized in a plurality of horizontal lines and wherein the combination of sub-fields for the particular pixel is selected on the basis of the combination of sub-fields selected for the pixel directly preceding the particular pixel on the same horizontal line. Now, only the combination of one pixel needs to be stored for determining the combination of sub-fields for the particular pixel. Furthermore, it is effective to determine the combination of sub-fields on the basis of the combination of the neighboring pixel since artifacts are often caused by errors between neighboring pixels.
An embodiment of the method according to the invention is to characterized in that the pixels of the image are organized in a plurality of horizontal lines and wherein the combination of sub-fields for the particular pixel is selected on the basis of the combination of sub-fields selected for the pixel located at the same position as the particular pixel on the horizontal line directly preceding the horizontal line of the particular pixel. In this embodiment, one horizontal line of the image needs to be stored, making it possible to determine the combination of sub-fields on the basis of the combination of a neighboring pixel on the previous horizontal line. It is effective to determine the combination of sub-fields on the basis of the combination of the neighboring pixel since artifacts are often caused by errors between neighboring pixels. Storing one horizontal line of the image only requires a moderately sized memory.
An embodiment of the method according to the invention is characterized in that the method comprises a step of determining edges of an area comprising the particular pixel and wherein the combination of sub-fields for the particular pixel is selected on the basis of the combination of sub-fields selected for at least one other pixel in that area. By analyzing the pixels of a certain area, it is possible to select that combination of sub-fields that will give the smallest chance on artifacts in that area.
It is a further object of the invention to provide an image display unit as described in the preamble with an improved reduction of artifacts. This object is achieved, according to the invention, in an image display unit which is characterized in that the selection means is arranged to select the combination of sub-fields for the particular pixel on the basis of the combination of sub-fields selected for at least one other pixel of the image. This allows the selection of that combination of sub-fields from the available combinations of sub-fields, which is most suitable regarding the actual content of the current image.